U.S. patent application number 12/430345 was filed with the patent office on 2009-12-24 for in-situ monitoring for laser ablation.
This patent application is currently assigned to Applied Materials, Inc.. Invention is credited to Wei-Yung Hsu, ANTOINE P. MANENS.
Application Number | 20090314752 12/430345 |
Document ID | / |
Family ID | 41319280 |
Filed Date | 2009-12-24 |
United States Patent
Application |
20090314752 |
Kind Code |
A1 |
MANENS; ANTOINE P. ; et
al. |
December 24, 2009 |
IN-SITU MONITORING FOR LASER ABLATION
Abstract
In a system where scribe lines are formed by a series of
partially-overlapping ablation spots, discontinuities can be
detected by capturing an intensity of light generated during each
instance of ablation for a respective spot. In any instance where
the intensity of light given off falls below a desired threshold,
such that the ablation spot might not sufficiently overlap any
adjacent spot, the position of that instance can be captured such
that another attempt at ablation can be carried out at that
location.
Inventors: |
MANENS; ANTOINE P.;
(Sunnyvale, CA) ; Hsu; Wei-Yung; (Santa Clara,
CA) |
Correspondence
Address: |
TOWNSEND AND TOWNSEND AND CREW LLP
TWO EMBARCADERO CENTER, EIGHTH FLOOR
SAN FRANCISCO
CA
94111-3834
US
|
Assignee: |
Applied Materials, Inc.
Santa Clara
CA
|
Family ID: |
41319280 |
Appl. No.: |
12/430345 |
Filed: |
April 27, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61053153 |
May 14, 2008 |
|
|
|
Current U.S.
Class: |
219/121.69 ;
219/121.83 |
Current CPC
Class: |
B23K 26/03 20130101;
B23K 2103/172 20180801; G01N 21/718 20130101; B23K 26/40
20130101 |
Class at
Publication: |
219/121.69 ;
219/121.83 |
International
Class: |
B23K 26/03 20060101
B23K026/03; B23K 26/38 20060101 B23K026/38 |
Claims
1. A system for scribing a workpiece, comprising: a laser for
directing a series of laser pulses toward a plurality of
partially-overlapping positions on a layer of material on the
workpiece, each laser pulse capable of triggering ablation of the
layer of material at one of the positions; and a detector for
detecting an intensity of light generated during the ablation, the
intensity of light being indicative of the amount of ablation at
each respective position.
2. A system according to claim 1, wherein the detector comprises a
photodiode.
3. A system according to claim 1, wherein the detector comprises a
spectral analyzer able to detect material ablated from an adjacent
layer of material on the workpiece.
4. A system according to claim 1, further comprising a filter to
substantially prevent the detector from detecting light of a
wavelength of the laser.
5. A system according to claim 1, further comprising a shutter
disposed in the optical path of the detector, wherein the shutter
is closed during the firing of the laser.
6. A system according to claim 1, further comprising a controller
for directing the laser back to any position where the detected
intensity indicates an unacceptable amount of ablation.
7. A system according to claim 1, wherein the workpiece is moved
relative to the laser and further comprising a trigger-distribution
controller for synchronizing the directing of the laser pulses with
the movement of the workpiece.
8. A system according to claim 1, further comprising an imaging
device for detecting the presence of a defect at any position where
the detected intensity indicates an unacceptable amount of
ablation.
9. A system according to claim 1, further comprising an algorithm
for directing a series of laser pulses toward an additional
plurality of positions in order to eliminate discontinuities at any
position where the detected intensity indicates an unacceptable
amount of ablation.
10. A method of scribing a workpiece, comprising: directing a
series of laser pulses toward a plurality of partially overlapping
positions on a layer of material on the workpiece, each laser pulse
capable of triggering ablation of the layer of material at one of
the positions; and detecting an intensity of light generated during
the ablation, the intensity of light being indicative of the amount
of ablation at each respective position.
11. A method according to claim 10, further comprising analyzing
spectral components of material ablated from the workpiece in order
to detect material ablated from an adjacent layer of material on
the workpiece.
12. A method according to claim 10, further comprising directing
the laser back to any position where the detected intensity
indicates an unacceptable amount of ablation.
13. A method according to claim 12, further comprising re-ablating
where the detected intensity indicates an unacceptably low amount
of ablation.
14. A method according to claim 10, further comprising: capturing
an image of the workpiece where the detected intensity indicates an
unacceptable amount of ablation; and processing the image so as to
identify a workpiece defect.
15. A method according to claim 14, further comprising generating a
series of overlapping laser ablations so as to circumvent the
workpiece defect.
16. An article comprising a storage medium having instructions
stored thereon, which instructions when executed result in the
performance of the following method: directing a series of laser
pulses toward a plurality of partially overlapping positions on a
layer of material on the workpiece, each laser pulse capable of
triggering ablation of the layer of material at one of the
positions; and detecting an intensity of light generated during the
ablation, the intensity of light being indicative of the amount of
ablation at each respective position.
17. An article according to claim 16, wherein the method performed
further comprises analyzing spectral components of material ablated
from the workpiece in order to detect material ablated from an
adjacent layer of material on the workpiece.
18. An article according to claim 16, wherein the method performed
further comprises directing the laser back to any position where
the detected intensity indicates an unacceptable amount of
ablation.
19. An article according to claim 18, wherein the method performed
further comprises re-ablating where the detected intensity
indicates an unacceptably low amount of ablation.
20. An article according to claim 16, wherein the method performed
further comprises: capturing an image of the workpiece where the
detected intensity indicates an unacceptable amount of ablation;
and processing the image so as to identify a workpiece defect.
21. An article according to claim 20, wherein the method performed
further comprises generating a series of overlapping laser
ablations so as to circumvent the workpiece defect.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application No. 61/053,153, filed May 14, 2008. This application is
related to co-pending U.S. Provisional Patent Application No.
61/044,021, filed Apr. 10, 2008, entitled "Laser Scribing
Platform." Each of these applications is hereby incorporated herein
by reference.
BACKGROUND
[0002] Various embodiments described herein relate generally to the
ablation of materials, as well as methods and systems for ablating
such materials. These methods and systems can be particularly
effective in scribing workpieces such as single-junction solar
cells and thin-film multi-junction solar cells.
[0003] Current methods for forming thin-film solar cells involve
depositing or otherwise forming a plurality of layers on a
substrate, such as a glass, metal or polymer substrate suitable to
form one or more p-n junctions. An example of a solar cell has an
oxide layer (e.g., a transparent-conductive-oxide (TCO) layer)
deposited on a substrate, followed by an amorphous-silicon layer
and a metal back layer. Examples of materials that can be used to
form solar cells, along with methods and apparatus for forming the
cells, are described, for example, in co-pending U.S. patent
application Ser. No. 11/671,988, filed Feb. 6, 2007, entitled
"MULTI-JUNCTION SOLAR CELLS AND METHODS AND APPARATUSES FOR FORMING
THE SAME," which is hereby incorporated herein by reference. When a
panel is being formed from a large substrate, a series of scribe
lines is typically used within each layer to delineate the
individual cells.
[0004] In some systems, scribe lines are formed using a series of
pulses from a laser directed toward at least one layer on a
workpiece. Each pulse is directed to, and focused at, the one or
more layers to be ablated, with the pulse having sufficient
intensity to ablate a "spot" or substantially circular region or
trench in the one or more layers. The ablated material is directed
away from the workpiece in a "plume" of debris. Unfortunately, due
to a number of variable factors, not every spot in a scribe line is
properly formed. In some instances, such as may be due to the
occurrence of a defect in the workpiece and/or a defective laser
pulse, a spot might not even be formed. Such improperly-formed
spots can create discontinuities in the scribe lines, which can
reduce the efficiency of the overall solar-cell array. Further, in
solar panels where scribe lines are formed from a billion or more
ablated spots, it can be especially time consuming to attempt to
locate and correct any individual discontinuity.
[0005] Accordingly, it is desirable to develop systems and methods
that overcome at least some of these, as well as potentially other,
deficiencies in existing ablating, scribing, and/or solar-panel
manufacturing devices.
BRIEF SUMMARY
[0006] The following presents a simplified summary of some
embodiments of the invention in order to provide a basic
understanding of the invention. This summary is not an extensive
overview of the invention. It is not intended to identify
key/critical elements of the invention or to delineate the scope of
the invention. Its sole purpose is to present some embodiments of
the invention in a simplified form as a prelude to the more
detailed description that is presented later.
[0007] Systems for laser scribing a workpiece are provided that
include a detector for monitoring laser ablations. By monitoring
the light generated during an ablation, a system can gather data
that is indicative of the amount of ablation at each respective
position. The data can be used for a variety of purposes, such as
for quality control and/or remedial actions, such as reworking the
workpiece by re-ablating or otherwise repairing locations on the
workpiece where the data is indicative of a defect. The systems
provided can be especially beneficial when used during the
manufacture of solar cells, such as single-junction solar cells and
thin-film multi-junction solar cells.
[0008] In an embodiment, a system for scribing a workpiece is
provided. The system includes a laser for directing a series of
laser pulses towards a plurality of partially-overlapping positions
on a layer of material on the workpiece. Each laser pulse is
capable of triggering ablation of the layer of material at one of
the positions. The system further includes a detector for detecting
an intensity of light generated during the ablation, the intensity
of light being indicative of the amount of ablation at each
respective position.
[0009] In another embodiment, a method of scribing a workpiece is
provided. The method includes directing a series of laser pulses
toward a plurality of partially-overlapping positions on a layer of
material on the workpiece. Each laser pulse is capable of
triggering ablation of the layer of material at one of the
positions. The method further includes detecting an intensity of
light generated during the ablation, the intensity of light being
indicative of the amount of ablation at each respective position.
In another embodiment, an article is provide that includes
instructions stored thereon, which instructions when executed
result in the performance of the above described method.
[0010] For a fuller understanding of the nature and advantages of
the present invention, reference should be made to the ensuing
detailed description and accompanying drawings. Other aspects,
objects and advantages of the invention will be apparent from the
drawings and the detailed description that follows.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] Various embodiments in accordance with the present invention
will be described with reference to the drawings, in which:
[0012] FIG. 1 illustrates a perspective view of a laser-scribing
device that can be used in accordance with many embodiments;
[0013] FIG. 2 illustrates an end view of a laser-scribing device
that can be used in accordance with many embodiments;
[0014] FIGS. 3(a) and 3(b) illustrate an approach for scribing
longitudinal trim lines on a workpiece that can be used in
accordance with many embodiments;
[0015] FIG. 4 illustrates layers of a solar cell with scribe lines
that can be formed in accordance with many embodiments;
[0016] FIGS. 5(a) and 5(b) illustrate discontinuities in scribe
lines that can be addressed in accordance with many
embodiments;
[0017] FIG. 6 illustrates a configuration for ablating material
from a workpiece that can be used in accordance with many
embodiments;
[0018] FIG. 7 illustrates intensity peaks that can be used in
accordance with many embodiments;
[0019] FIG. 8 illustrates a configuration of a laser-scribing
device that can be used in accordance with many embodiments;
[0020] FIG. 9 illustrates a configuration for laser ablation that
can be used in accordance with many embodiments;
[0021] FIG. 10 illustrates spectral peaks that can be generated and
analyzed in accordance with many embodiments;
[0022] FIG. 11 illustrates a pattern for correcting a discontinuity
that can be used in accordance with many embodiments; and
[0023] FIG. 12 illustrates a control system that synchronizes the
position of laser ablations with the movement of a workpiece in
accordance with many embodiments.
DETAILED DESCRIPTION OF THE INVENTION
[0024] Systems and methods in accordance with various embodiments
of the present disclosure can overcome one or more of the
aforementioned and other deficiencies in existing approaches to
ablation and/or laser scribing. Various embodiments can provide for
improved process control, as well as the ability determine in-situ
the presence and location of discontinuities or improper ablation
regions. Devices in accordance with various embodiments are then
able to return to these locations to attempt to correct for
problems in the ablation process.
[0025] FIG. 1 illustrates an example of a laser-scribing device 100
that can be used in accordance with many embodiments. The device
includes a bed or stage 102, which will typically be leveled, for
receiving and maneuvering a workpiece 104, such as a substrate
having at least one layer deposited thereon. In one example, a
workpiece is able to move along a single directional vector (i.e.,
for a Y-stage) at a rate of up to and/or greater than 2 m/s.
Typically, the workpiece will be aligned to a fixed orientation
with the long axis of the workpiece substantially parallel to the
motion of the workpiece in the device, for reasons described
elsewhere herein. The alignment can be aided by the use of cameras
or imaging devices that acquire marks on the workpiece. In this
example, the lasers (shown in subsequent figures) are positioned
beneath the workpiece and opposite a bridge 106 holding part of an
exhaust mechanism 108 for extracting material ablated or otherwise
removed from the substrate during the scribing process. The
workpiece 104 typically is loaded onto a first end of the stage 102
with the substrate side down (towards the lasers) and the layered
side up (towards the exhaust). The workpiece is received onto an
array of rollers 110 and/or bearings, although other bearing- or
translation-type objects can be used to receive and translate the
workpiece as known in the art. In this example, the array of
rollers all point in a single direction, along the direction of
propagation of the substrate, such that the workpiece 104 can be
moved back and forth in a longitudinal direction relative to the
laser assembly. The device can include at least one controllable
drive mechanism 112 for controlling a direction and translation
velocity of the workpiece 104 on the stage 102.
[0026] As the substrate is translated back and forth on the stage
102, a scribing area of the laser assembly effectively scribes from
near an edge region of the workpiece to near an opposite-edge
region of the workpiece. In order to ensure that the scribe lines
are being formed properly, an imaging device can image at least one
of the lines after scribing. Further, a beam-profiling device can
be used to calibrate the beams between processing of workpieces or
at other appropriate times. In an embodiment where scanners are
used, for example, which drift over time, a beam profiler allows
for the calibrating of the beam and/or adjustment of beam position.
The stage, bridge, and a base portion can be made out of at least
one appropriate material, such as a base portion of granite.
[0027] FIG. 2 illustrates an end view 200 of such a device,
illustrating a series of laser assemblies 202 used to scribe the
layers of the workpiece. In this example, there are four laser
assemblies 202, each including a laser device and elements, such as
lenses and other optical elements, needed to focus or otherwise
adjust aspects of the laser. Each laser device can be any
appropriate laser device operable to ablate or otherwise scribe at
least one layer of the workpiece, such as a pulsed solid-state
laser. As can be seen, a portion of the exhaust 108 is positioned
opposite each laser assembly relative to the workpiece, in order to
effectively exhaust material that is ablated or otherwise removed
from the workpiece via the respective laser device. In many
embodiments, the system is a split-axis system, where the stage
translates the workpiece along a longitudinal axis. The lasers then
can be attached to a translation mechanism able to laterally
translate the lasers relative to the longitudinal axis. For
example, the lasers can be mounted on a support 204 that is able to
translate on a lateral rail 206 as driven by a controller and servo
motor. In some embodiments, the lasers and laser optics all move
together laterally on the support. As discussed below, this allows
shifting scan areas laterally and provides other advantages.
[0028] Each laser device can produce multiple effective beams,
through the use of elements such as beam splitters, that are useful
for scribing the workpiece. Each portion of the exhaust can cover a
scan field, or an active area, of the beams from a common laser
device in this example, although the exhaust could be further
broken down to have a separate portion for the scan field of each
individual beam. The device also can include substrate-thickness
sensors useful in adjusting heights in the system to maintain
proper separation from the substrate due to variations between
substrates and/or in a single substrate. Each laser can be
adjustable in height (e.g., along the z-axis) using a z-stage,
motor, and controller, for example. In some embodiments, the system
is able to handle 3-5 mm differences in substrate thickness,
although many other such adjustments are possible. The z-motors
also can be used to adjust the focus of each laser on the workpiece
by adjusting the vertical position of the laser itself.
[0029] FIGS. 3(a) and 3(b) illustrate an exemplary approach that
can be used to form longitudinal scribe lines on the workpiece. As
shown in the illustration 350 of FIG. 3(b), the workpiece can be
moved back and forth longitudinally and only one scribe line can be
formed at any given time for any laser-beam portion or scan field.
The position of the scan field can be adjusted at the end of each
line. Each scribe line can be formed by ablating material at each
of a sequence of locations along the scribe pattern during movement
of the workpiece, forming a line of overlapping spots, as shown in
the illustration 300 of FIG. 3(a). The spots overlap by an amount,
such as 25% by area, that ensures proper region isolation in a
layer, of between parts of a cell, while minimizing the number of
spots that must be formed in order to ensure acceptable throughput.
Various methods of calibrating scribing devices are known, which
can provide a level of control of the positioning of the spots on
the workpiece. In the case of thin-film solar-cell panels, a number
of different scribe lines can be used in different layers to
provide for proper isolation between layer regions of different
cells. FIG. 4 illustrates an example structure 400 of a set of
thin-film solar cells that can be formed in accordance with many
embodiments. In this example, a glass substrate 402 has deposited
thereon a transparent-conductive-oxide (TCO) layer 404, which then
has scribed therein a pattern of first scribe lines (e.g., scribe 1
lines or P1 lines). An amorphous-silicon layer 406 is then
deposited, and a pattern of second scribe lines (e.g., scribe 2
lines or P2 lines) formed therein. A metal back layer 408 then is
deposited, and a pattern of third scribe lines (e.g., scribe 3
lines or P3 lines) formed therein. The area between adjacent P1 and
P3 (including P2 there between) lines is a non-active area, or dead
zone, which is desired to be minimized in order to improve
efficiency of the overall array. Accordingly, it is desirable to
control the spot size and positioning during the scribing
process.
[0030] As discussed, each scribe line in many embodiments is formed
by creating an "overlapping" series of ablation spots that
desirably form a continuous segment. Certain errors or problems can
occur, however, which can cause the scribe lines to be
discontinuous. Discontinuities in the scribe lines are undesirable,
as they can significantly reduce the electrical isolation between
adjacent regions and thus decrease the overall efficiency of the
panel. As illustrated in the example 500 of FIG. 5(a), it is
possible that an ablation spot 502 is formed that is too small,
such that gaps are left between that spot and at least one adjacent
spot, or the spots do not overlap enough to provide sufficient
isolation. In other cases, the spot may be too large and may reduce
the efficiency of the solar cells by reducing the active area of
the adjacent cells. FIG. 5(b) illustrates another example 550
wherein certain ablation spots 552 were not formed at all in the
layer, such as may be the result of a defect in the workpiece or a
failure of the respective laser pulse to reach the desired focus
position with the necessary intensity for ablation.
[0031] FIG. 6 illustrates a configuration 600 for forming these
ablation spots that can be used in accordance with many
embodiments. A pulse from a laser 606 is directed and/or focused by
at least one optical element 608 through a substantially
transparent (at least to the wavelength of the laser pulse)
substrate 602 to the desired location in a layer 604 to be ablated.
In some embodiments, the laser is a pulsed, Q-switched laser that
operating with a frequency of about 30-150 kHz, operating at a
wavelength on the order of about 266 nm, 532 nm, or 1064 nm. The
layers of material are on the opposite side of the workpiece from
the laser, such that the laser pulses pass through the substrate
and ablate the layer(s) on the top side in this arrangement, thus
causing the material at the focus location of the layer to ablate
up and away from the surface. Typically, the laser is focused near
the interface between layers. A laser pulse of sufficient intensity
then causes the region to heat rapidly, causing a minor "explosion"
that projects or bursts material from the workpiece. The material
ablated from the surface generally forms a plume 610 of material,
which can be extracted by the exhaust system. The plume in many
embodiments has a duration on the order of about 1-3 .mu.s. The
"burst" is generally accompanied by a flash of light, such as a
1-10 mm high "spark," caused by the rapidly heated gas, which can
include white light as well as other spectral components. A trench,
in many embodiments forming a substantially circular region free of
material, is then formed in the area of the ablation.
[0032] As discussed, not every ablation occurs as desired, due to
factors such as defects, variations, etc. When the ablation occurs
as desired, the light given off with the burst of the plume,
resulting from the heated gas, falls within a given range of
intensity. When the ablation process is not intense enough to form
a spot of sufficient size, the intensity of light generated with
the burst will be below this desired range. Accordingly, an
ablation step that produces too large a spot will have an intensity
exceeding this range, and when no ablation occurs there will be no
intensity as there is no burst or associated "spark" generated.
Systems and methods in accordance with various embodiments utilize
a detector to measure the intensity of the spark generated for each
ablation position. By detecting the intensity at each ablation
position, the system can determine which positions were not
properly ablated, and can correct those positions as needed in
order to ensure proper formation of the scribe lines. In the
example of FIG. 6, light generated from the spark will travel back
down the optical path toward the laser 606, and can be at least
partially directed by an optical element 612 such as a partially
transmissive mirror to an inline detector 614. The detector can be
any appropriate detebtbr, such as a fast photodiode with a 10-15 ns
response time. An example of such a detector is a white light
spectrum PIN photodiode available from ThorLabs, Inc. of Newton,
N.J. Another example of an appropriate detector is a
photomultiplier tube (PMT). PMTs are extremely sensitive detectors
of light in the ultraviolet, visible, and near-infrared ranges of
the electromagnetic spectrum. PMTs multiply the signal produced by
incident light by as much as 100 million times, enabling single
photons to be detected individually when the incident flux of light
is very low. A PMT can be used to detect a weaker plume that may
require a detector more sensitive than a photodiode, such as for P2
and P3 plumes. TCO plumes (P1) can be detected using photodiodes.
Placing the detector inline allows the detector to be substantially
centered at all times with respect to the ablation spot. The
detector can be synchronized with the firing of the laser 606 to
capture the intensity of the burst at each time of ablation. In
some embodiments there are about 10 .mu.s between shots (or
detections of consecutive plumes), with one shot per plume, for
plumes that last about 1-3 .mu.s. The duration between shots can be
adjusted, but sufficient time can be left between plumes to allow
each plume to sufficiently dissipate and allow the subsequent plume
to be separately resolved. In some embodiments, gas can be flowed
along, across, or sufficiently near the ablation spot in order to
help disperse the material and thus reduce the lifetime of the
plasma.
[0033] In some embodiments, a filter (not shown) can be added that
will substantially prevent the detector from detecting light of the
wavelength of the laser, in order to get a better indication of the
intensity of the spark. As shown, a detector 616 can be placed in
other positions, such as on the side of the ablation, but such
positions can come with certain disadvantages in certain systems,
as material from the ablation may collect on the detector, or there
may be very little space in the scribing device in which to
position the detector, particularly in complex devices with
multiple ablation processes occurring concurrently in a compact
area. In some embodiments, a shutter can be used in the path to the
detector that is closed during firing of the laser.
[0034] The detector can be connected to, or in communication with,
a controller such as is described in co-pending Provisional Patent
Application Ser. No. 61/044,021, incorporated by reference above.
In some embodiments, the detector captures positions where the
intensity did not fall within the desired range. As illustrated in
the example intensity vs. time graph 700 of FIG. 7, a range of
desired intensity readings can be defined by a minimum-intensity
value 702 and a maximum-intensity value 704. In some embodiments,
the minimum-intensity value corresponds to the ablation threshold,
or the minimum intensity needed to ablate the material. A peak 706
that falls within this range will in general correspond to a proper
ablation resulting in an ablation spot of a size within a desired
range. For a peak 708 that falls below the minimum intensity, the
position can be recorded so that the device can return to the
location to attempt to remove any discontinuity. For a peak 710
that falls above the maximum intensity, the system can attempt to
adjust the intensity of the laser pulses to correct the amount of
ablation. In some embodiments the captured position information can
include coordinates of the system or workpiece. In some
embodiments, the position information can include data such as a
longitudinal count of the stage-drive motor and a lateral count of
an optics-mount driver, etc. Any of a number of different
approaches to recording position can be used as would be apparent
to one of ordinary skill in the art in light of the teachings and
suggestions contained herein.
[0035] The position information can be stored in any appropriate
location, such as in local or cache memory. In order to conserve
memory, the system may only record the position of intensity
readings that fall outside the desired range, instead of intensity
information for each point on a particular workpiece. A device
controller in one embodiment is then able to use the position
information to go back to the recorded positions of unacceptable
intensity and attempt to ablate the position again in order to
correct for the previous ablation attempt. In some embodiments, the
discontinuities are fixed after the entire workpiece is ablated as
desired. In other embodiments, the system can attempt to correct
discontinuities on the same scribe line, or even shortly after
discovering a discontinuity in order to minimize the travel time
needed to navigate back to the position. Such an approach further
allows any parameters to be adjusted to improve subsequent
ablation, instead of waiting until the workpiece is finished.
[0036] FIG. 8 illustrates a configuration 800 wherein a
beam-splitting element 806, such as a partially-transmissive
mirror, half-silvered mirror, prism assembly, etc., is used to
split a laser pulse from a single laser 802 along two beam paths
each to a separate scanner 810 to focus and/or position the pulse
to the desired position and layer on the workpiece. While FIG. 8
illustrates some basic elements of an example laser assembly that
can be used in accordance with many embodiments, it should be
understood that additional or other elements can be used as
appropriate. In this configuration, the pulse along each path
passes through a shutter 808 to control the shape of each pulse,
and then a beam expander 804 to adjust the cross-sectional area of
the pulse to be focused onto the workpiece. Each beam portion can
also pass through other appropriate elements, such as an
auto-focusing element to focus the beam portion onto a scan head
810. Each scan head can include at least one element capable of
adjusting a position of the beam, such as a galvanometer scanner
useful as a directional deflection mechanism. In some embodiments,
this is a rotatable mirror able to adjust the position of the beam
along a lateral direction, orthogonal to the movement vector of the
workpiece, which can allow for adjustment in the position of the
beam relative to the intended scribe position. The scan heads then
direct each beam concurrently to a respective location on the
workpiece. A scan head also can provide for a short distance
between the apparatus controlling the position for the laser and
the workpiece. Therefore, accuracy and precision is improved.
[0037] In many embodiments, each scan head 810 includes a pair of
rotatable mirrors 812, or at least one element capable of adjusting
a position of the laser beam in two dimensions (2D). Each scan head
can include at least one drive element 814 operable to receive a
control signal to adjust a position of the "spot" of the beam
within the scan field and relative to the workpiece. In one
example, a spot size on the workpiece is on the order of tens of
microns within a scan field of approximately 60 mm.times.60 mm,
although various other dimensions are possible. When a scanning
device or scan head is used, the controller can utilize positioning
information from the scan head, longitudinal stage, and/or lateral
drive platform to obtain the proper position information of each
ablation spot on the workpiece. An inline camera 816 can be used to
image the workpiece, for example, to image scribe lines and/or
ablation plumes/sparks.
[0038] Analyzing the light given off by an ablation spark also
provides a second level of process control. In addition to the
diameter of an ablation spot, for example, an error also can be
introduced when the laser is not properly focused so as to ablate
only the proper layer. For example, consider the illustration 900
of FIG. 9. In this example, the pulse from the laser is intended to
pass through the substrate 902 and bottom layer 904 and be focused
at the top layer 906, typically near the interface with the
underlying layer 904, in order to ablate a region in the top layer.
Occasionally, however, the laser is focused at an improper depth in
the workpiece, such as may be due to mechanical variations, defects
in the workpiece, etc. The laser intensity may also be too high,
etc. When this happens, the ablation may include material from
other layers. As shown in the figure, the ablation not only occurs
within the top layer 906, but a portion 908 of the underlying layer
904 is ablated as well. Such problems again can lead to problems
with efficiency of the panel, and in some cases can even cause a
cell not to function properly.
[0039] Systems and methods in accordance with many embodiments can
detect such an issue in an approach similar to that described
above, except instead of simply using a detector such as a fast
photodiode, a spectral analyzer 910 or other such device can be
used that is able to distinguish spectral components in the
ablation plume 912. For example, a solar cell as described might
have a metal back layer overlying an amorphous-silicon layer. In
such a case, the system would expect the spectral analyzer to
detect at least one peak 1002 in the spectral region(s) of the
material used for the metal back layer, such as shown in the
generic spectral graph 1000 of FIG. 10. The intensity of a peak
corresponding to the flash still can be measured to determine the
amount of ablation, as discussed above. In addition, however, the
spectral analyzer is also able to detect and distinguish other
peaks 1004 that might appear in the spectrum. Using the example
above, the spectral analyzer might be able to detect in the
spectrum the presence of silicon, or a silicon compound, which
would indicate that the underlying layer was also being at least
partially ablated. If the presence of material from a different
layer persists for a significant amount of time, it is likely that
the laser needs to be refocused and the problem is not simply due
to a defect in the workpiece. The spectral results can be
continually fed to a controller in some embodiments that is able to
adjust the focus on-the-fly in order to improve process
control.
[0040] As discussed above, detecting problems with ablation spots
can allow a device to go back, automatically or manually (or a
combination thereof), and attempt to ablate the location again
where the problem results in a discontinuity. Generally, this will
involve translating back to that position and re-attempting
ablation. Occasionally, however, the discontinuity will be due to a
defect in the workpiece such as an air bubble in the substrate or a
particle on a surface of the workpiece. In some cases, such a
defect might cause several sequential ablation spots to not be
formed properly. An approach 1100 to correcting discontinuities in
accordance with some embodiments is illustrated in FIG. 11. Shown
is a partial top view, showing a discontinuity in a scribe line
1104 formed by ablation spots, where the discontinuity overlies (or
underlies) an air bubble or other defect in the workpiece. In this
case, since the spots cannot be ablated due to the defect, a
pattern of ablation spots 1106 can be determined that will
circumvent the defect 1102. The determination of such a pattern can
be done manually, by notifying a user to optically inspect the
defect, or automatically through, for example, a camera and pattern
recognition software, etc. As shown, the pattern can allow the
scribe lines to be formed without discontinuities, and with a
minimum amount of dead space created. Of course, if the defect is
so large as to cover multiple cells, then there may be no way to
salvage all of the cells. Further, once a defect gets to a certain
size there may be more benefit to not spending time fixing the
discontinuity and just giving up the efficiency of a single cell,
etc.
[0041] Other process-control functions can further help improve the
quality of the final scribe lines. For example, during the scribe
process, an imaging device or profiler can image the pattern
scribed on the workpiece to ensure proper control of the pulsed
beam by the respective scan head. Further, while four lasers are
shown with two beam portions each for a total of eight active beams
in the examples, it should be understood that this is merely
illustrative and that any appropriate number of lasers and/or beam
portions can be used as appropriate, and that a beam from a given
laser can be separated into as many beam portions as is practical
and effective for the given application. Further, even in a system
where four lasers produce eight beam portions, fewer than eight
beam portions can be activated based on the size of the workpiece
or other such factors. Optical elements in the scan heads also can
be adjusted to control an effective area or spot size of the laser
pulses on the workpiece, which in some embodiments vary from about
25 microns to about 100 microns in diameter.
[0042] FIG. 12 illustrates a system 1200, in accordance with many
embodiments, that can be used to enhance scribe-placement accuracy
by synchronizing the stage-encoder pulses to the laser and
spot-placement triggers. The system 1200 can ensure that the
workpiece is in the proper position, and the scanners directing the
beam portions accordingly, before the appropriate laser pulses are
generated. Synchronization of all these triggers can be simplified
by using a trigger-distribution controller 1202, such as a single
VERSAmodule Eurocard (VME) controller, to drive all these triggers
from a common source. The trigger-distribution controller 1202
receives a trigger signal from the stage controller 1204, which is
used to control the movement of the workpiece via the multi-axis
laser-scribing stage 1206. The trigger-distribution controller
passes the trigger signal to the laser and scanner controllers
1208. The laser and scanner controllers 1208 use the trigger signal
to synchronize the scanning of the laser and the switching of the
laser via laser scanner 1210 and laser source (Q-switch) 1212,
respectively. Various alignment procedures can be followed for
ensuring alignment of the scribes in the resultant workpiece after
scribing. Once aligned, the system can scribe any appropriate
patterns on a workpiece, including fiducial marks and bar codes in
addition to cell delineation lines and trim lines.
[0043] The specification and drawings are, accordingly, to be
regarded in an illustrative rather than a restrictive sense. It
will, however, be evident that various modifications and changes
may be made thereunto without departing from the broader spirit and
scope of the invention as set forth in the claims.
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